WO1998020691A1

WO1998020691A1 - Method for frequency planning in wireless local loop communication system

Info

Publication number: WO1998020691A1
Application number: PCT/US1997/019416
Authority: WO
Inventors: John R. Doner
Original assignee: Airnet Communications Corporation
Priority date: 1996-11-01
Filing date: 1997-10-30
Publication date: 1998-05-14
Also published as: AU5089398A; WO1998020685A2; AU5096698A; US5901355A

Abstract

A Frequency allocation technique for a wireless system which employs remote subscriber Field Access Units (FAUs, 20-a, 20-d) that use omni-directional antennas in an inner region of a cell, and directional antennas in an outer region of the cell. Different frequency subsets are used for the inner and outer cell regions and FAUs located in the inner regions of homologous cells maintain separation from one another by limiting their operating power to a level needed to complete the radio link. A receiver portion of the base station (12) has the capability to determine received signal power for each channel. This provides the basic input for a channel selection algorithm which determines the quietest channel from among those channels not in use. A further constraint on the frequency allocation process is that a minimum number of channels always remains unused. That is, for example, among the available channels in each cell, only a subset of the channels are actually ever allowed to become active.

Description

METHOD FOR FREQUENCY PLANNING IN WIRELESS "LOCAL LOOP

COMMUNICATION SYSTEM

FIELD OF THE INVENTION

This invention relates generally to wireless communications and in

particular to a technique for assigning frequencies to mobile units according to

a reuse pattern adapted for Wireless Local Loop (WLL) applications.

BACKGROUND OF THE INVENTION

The increasing demand for wireless communication services such as

provided by cellular and Wireless Local Loop (WLL) systems requires the

operators of such systems to attempt to make a maximum effective use of the

available radio frequencies. Consider, for example that a WLL system operator

is typically allocated a geographic territory and a certain amount of radio

bandwidth that affords the ability to transmit and receive on only a particular

number of radio channels. In an effort to make the best use of the allocated

frequency space the geographic territory is divided into a number of sub-areas called cells. A number of base stations are deployed throughout the assigned

territory, with there typically being one base station in each cell. Transmission

power levels are kept low enough so that the subscriber units, called Fixed

Access Units (FAUs), in adjacent cells do not interfere with each other.

The system operator can then determine how to split up the allocated

radio frequencies among the cells so that FAUs using the same frequencies do

not interfere with one another. This process is intended to maximize channel

availability in the service area, that is, to maximize the number of channels

which may be used in a particular area at any one time. The object of this

frequency planning process is to reuse each frequency as often as possible.

Cells which reuse the same frequency set in this manner are referred to as

homologous cells. In general, reusing a frequency in every Nth cell thus means

that 1 N of all frequencies are available in any given cell. It is therefore usually

desirable to select the reuse factor, N, to be as small as possible in order to

increase the capacity for handling remote units in each cell.

However, conflicting with this requirement is a real world consideration

that as the reuse factor N is decreased, the interference between channels in

homologous cells increases. In other words, there is a design dilemma in that as N decreases, so does the distance between homologous cells^", and thus the

amount of interference between FAUs located in different cells but operating

on the same frequency increases. The ratio between the RF power of a desired

carrier signal, C, and the interference, I, created by FAUs operating in

homologous cells is referred to as the carrier-to-interference ratio, C/I. Thus, as

the reuse factor N decreases, it is generally understood that the C/I ratio is

normally expected to increase.

What is needed is a way to decrease the channel reuse factor by

partitioning the use of frequencies among cells without also necessarily

imposing an increase in the carrier to interference ratio.

DESCRIPTION OF THE INVENTION

Summary of the Invention

Briefly, the invention is a Wireless Local Loop (WLL ) system which

employs remote subscriber Field Access Units (FAUs) that use both directional

and omni-directional antennas. The omni-directional antennas are used in an

inner region of the cell, out to a specific radius, r, from the center of the cell.

Directional antennas are used in an outer region of the cell, from the specific

radius, r, out to the cell boundary radius, R. Optimally, the ratio between the

inner radius, r, and outer radius, R, is chosen to balance the carrier-to-

interference (C/I) ratio to be about the same on average for FAUs in both the

inner and outer regions.

In addition, different frequency subsets are used for the inner and outer

cell regions, in order to reduce intra-cell interference. The FAUs located in the

inner regions of homologous cells maintain separation from one another by

limiting their operating power to a level needed to complete the radio link from

the base station to the FAU, in both the uplink and downlink directions. A receiver portion of the base station has the capability to determine

received signal power for each channel within the bandwidth being served.

This provides the basic input for a channel selection algorithm which

determines the quietest channel from among those channels not in use. Thus,

as newly activiated FAUs require an operating frequency to be assigned, the

quietest channel among those available is first assigned.

A further constraint on this frequency allocation process is that a

minimum number of channels always remain unused. That is, for example,

among the available channels in each cell, only a subset of the channels are

actually ever allowed to become active.

The results of a computer simulation of a WLL system employing

directional antennas and the above frequency assignment algorithm has

indicated that with an N=2 reuse pattern, it is possible to provide a higher

carrier-to-interference ration (C/I) than with an N=3 reuse pattern operating

with random channel selection.

In other words, an advantage of the invention is that a two cell reuse

pattern with channel use in the manner specified by the invention can provide greater C/I and more active channels per cell than can be achieved by using a

standard narrow band channel radio processing and random assignment

schemes. In a specific example, up to 62 channels per cell can be allocated in

an Advanced Mobile Phone Service (AMPS) type wireless local loop

application with a 5 MHz radio allocation in accordance with the invention,

whereas the same 5 MHz of spectrum would normally only support 22 channels

per cell with a standard frequency reuse scheme and random channel

assignment.

Brief Description Of The Drawings

For a more complete understanding of the invention and its novel

advantages and features, reference should be made to the accompanying

drawings in which:

Fig. 1 is a block diagram of a wireless local loop (WLL) system in

which the invention may be employed;

Fig. 2 is a block diagram of one preferred arrangement of the base

stations shown in Fig. 1 to obtain signal strength measurements;

Fig. 3 is an illustration of a cell plan layout in which each cell has an

inner omni-directional region and outer directional region;

Fig. 4 illustrates an N=2 frequency reuse plan in accordance with the

invention showing frequency assignments to inner and outer regions of each

cell; and

Fig. 5 is a flowchart of operations performed by a base station or base

station controller in order to assign channels to Fixed Access Units (FAUs). Detailed Description of a Preferred Embodiment

Turning attention now to the drawings, Fig. 1 illustrates a Wireless

Local Loop (WLL) communication system 10 in which the invention may be

advantageously used. The system 10 includes a plurality of base stations 12

with each base station 12 being associated with a sub-area, or cell, of a

geographical territory assigned to a wireless service provider. A number of the

base stations, 12-1, 12-2, ..., 12-n, are typically arranged in a group, or cluster.

A base station controller 14 and telephone switching center 16 provide

connections between the base stations 12 and a public switch telephone

network (PSTN) 18.

The system 10 typically provides multiple Fixed Access Units (FAUs)

20-o and 20-d at least standard voice quality communication service,

supporting the ability to communicate among each other or with other devices

that may be connected to the PSTN 18. The Base Station Controller (BSC) 14

is responsible for coordinating these connections by controlling the operation of

the base stations 12 and the switching center 16 to set up the appropriate

transmission and reception of voice (or possibly data) and control signals

between FAUs 20 and the PSTN 18. For example, when an FAU 20 first requests service it transmits a control signal which is received by one of the

base stations 12. This control signal is forwarded through the base station

controller 14 and to the switching center 16. The switching center 16 then sets

up a voice or data connection to the PSTN 18, while the base station controller

14 determines a radio frequency assignment for the FAU 20 requesting access

and arranges for radio transceivers located within the base station to handle the

over-the-air communications with the FAU 20.

The present invention lies in the manner in which a frequency

assignment is determined for the FAUs 20. Although the following detailed

description will describe an embodiment in which frequency assignment is

carried out in the base station controller 14, it should be understood that in

some wireless system architectures, base station controllers 14 may not be

present and/or it may otherwise be desirable to carry out the frequency

assignment within processors located in the base station 12 or even with

processors located in the switching center 16.

The invention is specifically adapted to provide efficient frequency

assignments in systems for which the base stations 12 make use of broadband

radio transceivers which can transmit and receive on multiple radio frequency carriers at the same time while providing information concerning the receive

signal strength across a broadband of operating frequencies.

The base station 12 consists of a receive antenna 21, one or more

broadband digital tuners 22, one or more digital channelizers 24, a bit-parallel

Time Division Multiple (TDM) sample bus 26, a plurality of digital signal

processors (DSPs) 28 programmed to operate as demodulators and modulators,

an interprocessor communication mechanism 30, a transport signal (T-l)

encoder 31, a T-l decoder 32, one or more digital combiners 34, one or more

broadband digital exciters 36, a power amplifier 38, a transmit antenna 39, a

control processor 40, and a synchronization clock generator 42.

Briefly, on the receive side (that is, with respect to the base station 12),

radio frequency (RF) carrier signals from the FAUs 20 are first received at the

receive antenna 21 and then forwarded to the broadband digital tuner 22. The

digital tuner 22 down converts the RF signals to an intermediate frequency (IF)

and then performs an analog to digital (A/D) conversion to produce a digital

composite signal 23. The digital channelizer 24 implements a filter bank to separate the

composite digital signal 23 into a plurality, n, of individual digital channel

signals 25-1, 25-2, ..., 25-n (collectively, channel signals 25). The digital

channelizer 24 may implement the filter bank using any of several methods as

is known in the art. The samples which comprise the n digital channel signals

25 are then provided over the TDM bus 26 to the DSPs 28. A subset of the

DSPs 28 are programmed to remove the modulation on the channel signals 25

as specified by the air interface standard in use. As part of this process, the

DSPs 28 determine a receive signal strength for each of the n digital channel

signals and then periodically provide this information to the control processor

to which in turn forwards the receive signal strength information to the base

station controller 14.

The demodulated signal outputs of the DSPs 28, representing baseband

audio or data signals, are then forwarded over the communication mechanism

30 to the encoder 31 , which in turn reformats the demodulated signals as

necessary for transmission over a land based telephone line forwarding such to

the switching center 16 (Fig. 1).

The signal flow on the transmit side of the base station 12 is analogous. For a more detailed description of such a system please refer to co-

pending U.S. patent application entitled "Wideband Wireless Base Station

Making Use of Time Division Multiple Access Bus to Effect Switchable

Connections to Modulator/Demodulator Resources", serial number 08/251, 914

filed June 1, 1994 which is assigned to AirNet Communications Corporation,

the assignee of this application.

In order to set up each call, the control processor 40 in the base station

12 must exchange certain control information with the base station controller

14. For example, when an FAU 20 wishes to place a call, it indicates this by

transmitting on one or more control signal channels. These control signals may

be in band or out of band signals present in one or more of the channel signals

output by the channelizer 24 for input to the combiner 34. If out of band, a

separate control signal transceiver 45 may be used to receive and transmit such

control signaling.

In any event, the control signals are forwarded to the control processor

40 which requests the BSC 14 (Fig. 1) to set up the end to end connection

through the switching center 16. Upon receiving an indication from the

switching center 16 that the connection can be made at the remote end, the base station controller 16 then performs a number of steps to insure- that the

appropriate signal path through the base station 12 is enabled. Of particular

interest in this case is the need to determine a pair of transmit and receive radio

frequencies to be used for the call. As mentioned previously, the present

invention lies in the details of how frequencies are allocated by the base station

controller 14 for use among the various base stations 12. The following

description will explain how an FAU transmit frequency (i.e., in the uplink, or

FAU 20 to base station 12 direction) is determined. The receive frequency (i.e.,

in the downlink or base station 12 to FAU 20 direction), is typically determined

as a constant offset from the FAU transmit frequency.

The FAUs 20 are stationary terminals and thus the WLLs system 10

shown in Fig. 1 permits the use of directional antennas for certain FAUs such

as shown for FAU 20-d. Although directional antennas do add initial cost to

the system and require a more precise installation of the FAUs 20-d, three

advantages do occur. First, the greater gain provides a dramatic increase in the

maximum cell radius. Second, the FAU 20-d receives less interference from

directions other than in the direct line of sight from the base station 12. Third,

the base station 12 receives less interference from other FAUs 20 because transmit energy from the directional FAU 20-d is highly concentrated along this

line of sight access.

In accordance with the invention however, not all FAUs 20 are provided

with directional antennas. Fig. 3 illustrates the situation as contemplated with

the use of both omni-directional FAUs 20-o located within an inner region 60

within a specified radius r of the base station 12. The directional antenna FAUs

20d are used in an outer region 62 beyond the radius r out to a radius R which

represents the cell boundary. Optimally, the ratio between the inner radius, r,

and outer radius, R, is chosen to balance the carrier-to-interference (C/I) ratio to

be about the same on average for FAUs in both the inner and outer regions.

In such a cell there must still be assurance that the omni directional

FAUs 20-o are operating with adequate carrier-to-interference (C/I) ratio. In

particular, even if the system is originally planned to provide adequate C/I for

directional FAUs 20-d in the outer region 62, simply doing this alone is

incompatible with providing adequate C/I for the omnidirectional FAUs 20-o

employed in the inner region 60. This difficulty is resolved by using separate

frequency subsets for the inner region 60 and outer region 62, and by power

control on the omnidirectional FAU 20-o uplink and downlink, and by assigning different frequency subsets for the inner 60 and outer regions 62, the

inner regions 60 in adjacent cells therefore are assured of maintaining some

distance from one another. By operating with a limited power level which is

only needed to complete the link out to the distance r, the overall interference

situation for the inner region 60 will be as though the inner region 60 were

operating at a much higher frequency reuse factor than is actually employed.

Fig. 4 illustrates the situation more particularly, showing a deployment

of base stations for a frequency reuse factor of N=2. Only homologous cells

are shown in detail, for the sake of clarity. In this scenario, like cells are laid

out in a "checkerboard" fashion. Although the outer regions 62 are shown as

being circular, they are typically not perfect circles but rather, the exact shape

of course depending upon the number of factors including the surrounding

terrain features.

Since two frequency sets are needed in each cell, for the inner 60 and

outer region 62, and since N=2, the available frequencies are thus divided into

four sets. These include sets II and 12 assigned for use in the inner regions 60,

and sets 01 and 02 assigned for use in the outer regions 62. Frequency subset

assignments are made such that adjacent cells use different frequency subsets in both the inner and outer regions. For example, a cell 64-1 useδ the frequency

set II in the inner region 60-1 and set 01 in its outer region 62-1. An adjacent

cell 64-2 uses frequency set 12 in its inner region 60-2 and set 02 in its outer

region 62-2.

Although the channels are available for use in each region of a cell in the

example being discussed, the channel availability is derated, such as to permit

for example, only a certain number of channels in sets II and 12 to be active in

the inner region 60, and to permit only a certain number of channels in sets 01

and 02 to be active in the outer region 62. By "derating" the available number

of channels in this fashion, it has been determined that the C/I ratio can be

increased remarkably.

As previously mentioned the channel selection algorithm makes further

use of the broadband digital tuner 22 and digital channelizer 24, which provide

a measure of the signal energy in each of the 166 channels across the available

5 MHz bandwidth to the base station controller 14.

Now more particularly, the flow diagram of Fig. 5 illustrates the process

of assigning a frequency to a new FAU 20 as it comes on line. In a first step 100, the available signal strengths for the channels in each of the cells are

periodically measured by the digital channelizer 24 and reported by the base

station 12 to the base station controller 14. In step 102 the base station

controller 14 updates a list of quietest unused channels for each of the inner and

outer portions of the particular cell. This is, for example, a pair of lists of

unused channels referred to herein as the I-unused list and the O-unused list.

The I-unused channel list and O-unused channel list are taken from either the II

and 01, or 12 and 02 subsets respectively, assigned to the particular cell. The

I-unused and O-unused lists are kept in a memory portion of the base station

controller 14 (Fig. 1).

In step 104 which is periodically entered into, it is determined whether

or not a request to permit an FAU 20 to access the system 10 has been received.

If the request is for the FAU 20 to be removed from the active list, control

proceeds to step 105 where the channel which was presently in use is released

and returned to either the I-unused or O-unused list of free channels, depending

upon where the FAU is located. If however, in step 104, a new FAU 20

requires access to the system 10, control passes to step 106 where it is determined if the FAU 20 is in the inner region 60 or outer region 62 of the cell.

If the FAU 20 is in the inner region 60, control passes to step 108.

In step 108, the number of inner channels presently in use is examined.

If less than a predetermined number, I, of the inner channels have not yet been

used, control passes to step 110 where the quietest of the available channels on

the I-unused list is assigned for use by the new FAU 20.

If however, in step 108 no inner channels are available for use, that is,

the predetermined number I of inner channels are already in use, control passes

to step 112 where access is denied to the FAU 20.

In step 106, the FAU 20 requesting access is located in the outer region

62 of the cell, and control passes to step 114 where it is first determined if the

number of outer channels in use exceeds a pre-determined number, O. If this is

the case, then control passes to step 116 wherein the quietest of the available

outer channels is assigned for use by the FAU 20. If the maximum number, O,

of outer region 62 channels has already been assigned, then control passes to

step 118, where the FAU 20 is not permitted to operate. A specific example of implementing the invention for use with the

analog Advance Mobile Phone Service (AMPS) protocol will now be

described. Consider the case of a 5 MHz spectral allocation per cell. In each

cell are thus available 166 channels, each with a 30 kHz standard AMPS

bandwidth. Assuming that fourteen (14) control channels are reserved, at least

two control channels per cell for the inner 60 and outer regions 62 in a standard

seven cell reuse pattern. This leaves 152 total channels for communication

channel use. Given N=2, then 76 channels are available for use in each cell.

The 76 channels per cell are then each divided into the two sub-sets, sets II or

12 for supporting the inner portion of the cell, and the other set 01 or 02 for

supporting the outer portion of the cell.

The invention was subjected to a computer simulation in which the

seventy-six (76) channels per cell were de-rated to permit each base station to

use sixty-two (62) channels. Twenty-four (24) channels were allocated to the

omni-directional inner region 60 and thirty-eight (38) channels allocated for use

in the directional outer region 62. The FAUs 20 were assumed to be uniformly

distributed with the cell having an inner region 60 with a radius r of 50% of the

total cell radius R, thereby having approximately 40% of the cell area and channels to be devoted to the inner region 60 and approximately 60% of the

area and available channels to using the directional antennas in the outer region

62. The simulation indicated mean and 99th percentile carrier-to-interference

(C/I) ratios as follows:

Receive Link Mean C/I 99th percentile C/I

BTS inner region 30 dB 18 dB

FAU inner region 29.6 dB 18 dB

BTS outer region 30.3 dB 19 dB

FAU outer region 29.4 dB 19 dB

Simulation results for an N=3 reuse pattern with random channel

selection and no derating were:

Receive Link Mean C/I 99th Percentile C/I BTS inner region 29 dB 24 dB FAU inner region 29.2 dB 24 dB

BTS outer region 30 dB 22 dB FAU outer region 29.4 dB 23 dB

The simulations thus indicated that the N=2 cell reuse pattern indicated

in Fig. 4 with quietest channel de-rating can provide a higher mean C/I ratio

than an N=3 cell frequency reuse plan operating with simple random channel selection. In other words, the frequency assignment scheme can be used to

advantage to provide a better C/I ratio, and in effect allow more channels per

cell to operate than could otherwise be achieved when a wireless system 10 is

implemented using narrowband transceivers and conventional channel

assignment schemes.

The foregoing description has been limited to specific embodiments of

this invention. It is apparent, however, that variation and modifications may be

made to the invention as described above with the attainment of some or all of

its advantages.

What is claimed is:

Claims

1. A wireless system for operation in a service area which is divided

into cells, comprising:

a plurality of remote subscriber Field Access Units, FAUs, located

throughout the service area, such that the FAUs use omni-directional antennas

in an inner region of the cells, and such that the FAUs use directional antennas

in an outer region of the cells;

a plurality of base stations located in the cells, for transmitting and

receiving radio signals from the FAUs, and limiting the operating power of

such radio signals transmitted to and received from FAUs located in the inner

regions of the cells to a power level needed to complete a link between the base

station and the FAU, and the base stations having the capability to determine a

received signal power value for each available operating radio frequency in the

system;

channel selection means, connected to receive the signal power values

from the base stations, for selecting operating radio frequencies for the FAUs in each cell by keeping a list of quietest channels by examining the received signal

power values from among those operating radio frequencies not presently in

use, and ensuring that a minimum number of operating radio frequencies

remain unused such that among the unused operating radio frequencies in any

given cell, only a subset of the operating radio frequencies are ever used.

2. A wireless system as in claim 1 wherein the inner region of a cell is

selected as that portion of the service area located within an inner radius, r,

about the center of the cell.

3. A wireless system as in claim 2 wherein the outer region of a cell is

selected as that portion of the service area located between the inner radius, r,

and an outer radius, R, located near an outer boundary of the cell.

4. A wireless system as in claim 3 wherein the inner radius, r, and outer

radius, R, are chosen such that the average carrier-to-interference (C/I) ratio is

the same on average for signals received by FAUs located in both the inner and

outer regions.

5. A wireless system as in claim 1 wherein different subsets of

operating radio frequencies are made availabe for use for the inner and outer

regions.

6. A wireless system as in claim 5 wherein a frequency reuse factor of

two is used among the cells such that four radio frequency sets are allocated

among the cells, such that a first pair of radio frequency sets, II and 12, are used

in the inner regions of adjacent cells and such that a second pair of radio

frequency sets, 01 and 02, are used in the outer regions of adjacent cells.

7. A method of assigning a frequency to a remote unit in a wireless

communication system, the wireless communication system employing a

number of base stations, the remote units which are located in an inner region

of the cells using omni-directional antennas, and the remote units which are

located in an outer region of the cells using directional antennas, and wherein

each cell has at least two groups of frequencies assigned to it so that different

frequencies are used for the remote units located in the inner region of a cell

that are used in the outer region of the same sell, and such that there are two

different groups of frequencies assigned to adjacent cells, the method comprising the steps of:

determining when a new remote unit is requesting service from the

communication system;

detecting the region of the cell within which the remote unit is located

by measuring a signal strength indication, RSSI, of a radio signal received from

the new mobile unit at each of the sector antennas;

detecting whether the remote unit is located in the inner region of the

cell the outer region of the cell, by comparing the RSSI to a predetermined

threshold value;

if the remote unit is located in an inner region of the cell, assigning a

frequency to the remote unit which is not in use in the outer region of the cell.

8. A method as in claim 7 additionally comprising the steps of:

keeping a list of quietest channels by examining the RSSIs from among

those radio frequencies not presently in use, and ensuring that a minimum number of radio frequencies remain unused in each cell such that among the

unused operating radio frequencies in any given cell, only a subset of the radio

frequencies are ever used.